1. Field of the Invention
The present invention relates to a non-aqueous secondary electrochemical battery comprising a complex oxide containing lithium for a cathode and carbon for an anode, and more particularly, to a non-aqueous secondary electrochemical battery having improved cycle life capabilities, discharge performance, and low temperature performance.
2. Description of the Related Art
Recently, various kinds of portable or cordless electronic equipment have been developed one after another, and as a power source for driving these equipments, the demand for small-sized and lightweight secondary batteries which have high energy density has increased. In this respect, because of their high voltage and high energy density, non-aqueous secondary lithium batteries have been desired.
As for secondary batteries, nickel-cadmium batteries and lead acid batteries having excellent performance capabilities are commercially available. Therefore, when non-aqueous electrochemical batteries are used as secondary batteries, it is desired that cathode active materials for these batteries have high energy density, that is, high capacity and high potential.
As a cathode active material, a complex oxide containing lithium is well known. For example, U.S. Pat. No. 4,357,215 discloses a battery comprising LiCoO.sub.2 as an active material for a cathode.
On the other hand, U.S. Pat. No. 4,423,125 discloses a non-aqueous electrochemical battery which comprises carbon for an anode instead of lithium metals or lithium alloys. Since this battery uses carbon capable of intercalating and deintercalating lithium ions, it exhibits safety and good cycle life capability.
Moreover, Japanese Laid-Open Patent Publication No. 63-121260 discloses a combination of these electrodes, in which LiCoO.sub.2 and carbon are used for a cathode and an anode, respectively.
When carbon such as graphite is used for an anode, it is required that a complex oxide containing lithium, e.g., LiCoO.sub.2, is used as a cathode and as a lithium source. Since a lithium metal is not used for the anode, active dendritic products, that is, so-called dendrites are not produced on the surface of the anode during the charge, while on the other hand dendrites are produced when a lithium metal is used for an anode. As a result, the cathode and anode are kept free from the penetration of the dendrites through a separator, which would otherwise cause a short circuit therebetween. The battery can be prevented from igniting or exploding. In this way, the secondary battery which is safe and excellent in discharge-charge cycle life capabilities can be obtained. However, the discharge-charge cycles involve a decomposition of a solvent of a non-aqueous electrolyte as a side reaction, and repetition of the cycles gradually deteriorates the characteristics of the battery.
When a lithium metal is used for an anode, after repetition of the charge-discharge cycles, dendrites are produced on a surface of the anode during the charge not only to cause a short circuit but also to react with a non-aqueous solvent to partially decompose the solvent during the charge. As a result, charge efficiency is lowered. In this system, the maximum charge efficiency is approximately in the range of 98 to 99%. When a lithium alloy is used for the anode, although the dendrites are not produced, the non-aqueous electrolyte is decomposed on the surface of the alloy during the charge. As a result, the maximum charge efficiency is said to be approximately 99%.
When the carbon is used for the anode, lithium, an active material, is intercalated between layers of the carbon. Therefore, the decomposition of a solvent on the surface of the anode as mentioned above should not be caused. However, the charge efficiency of 100% can not actually be attained.
It is assumed by the inventors that the reason for the above-mentioned side reaction with the carbon anode is as follows:
When carbon is used for the anode, lithium ions and a solvent are co-intercalated between the carbon layers. At that time decomposition of the solvent sometimes occurs. That is, the solvent whose molecular diameter is large can not be intercalated between the layers, so that the solvent is partially decomposed at the entrance of the carbon layers.
Examples of a solvent for an electrolyte of the above-mentioned lithium battery preferably include esters such as propylene carbonate and ethylene carbonate. U.S. Pat. No. 4,804,596 also discloses that an ester-based electrolyte is preferably used when LiCoO.sub.2, one of the lithium containing oxides, is used for a cathode.
One of the requirements for a solvent suitable for a lithium battery is a high dielectric constant, that is, a capability of dissolving a large amount of inorganic salt which is a solute. The above-mentioned propylene carbonate and ethylene carbonate satisfy this requirement. However, these esters have cyclic structures and large molecular diameters. Therefore, when a lithium ion and a solvent are co-intercalated between the carbon layers, this type of solvent is partially decomposed during the charge as described above.
On the contrary, solvents having a chain structure such as chain esters are readily intercalated between the carbon layers because of their structure. However, dimethylformamide and acetonitrile, which are known to have a high dielectric constant for dissolving a large amount of an inorganic salt, are highly reactive to lithium. Therefore, it is difficult to use them practically although they are easily intercalated between the layers. Among the chain esters, diethyl carbonate and ethyl acetate are not reactive to lithium and easily intercalated between the carbon layers, but they have a low dielectric constant, so that they are unable to dissolve a large amount of an inorganic salt.
To solve the above-mentioned problems, the inventors of the present invention discovered a use of a mixed solvent containing a cyclic ester and a chain ester as a solvent for an electrolyte, whereby a large amount of an inorganic salt is dissolved and a lithium ion with a chain ester is readily intercalated and deintercalated between the layers of the carbon without the decomposition of the solvent.
Further, it is found in this application that among the chain esters, an asymmetric chain carbonate provides particularly excellent characteristics of a battery.
Japanese Laid-Open Patent Publication No. 2-148665 already discloses the use of an asymmetric chain carbonate as a solvent suitable for an electrolyte of a lithium secondary battery having another electrode system. According to the present invention, however, a mixture of an asymmetric chain carbonate and a cyclic ester used for a carbon anode provides a specific effect.
More specifically, there is a specific correlation between the carbon for the anode and the cyclic ester to be mixed with the asymmetric chain carbonate. When graphites such as an artificial graphite, natural graphite or a mesocarbon microbead having been heat-treated at a high temperature of 2000.degree. C. or more having a lattice spacing (d002) of 3.40 angstroms or less, which can be measured by a wide angle X-ray diffraction method of carbon, are used, ethylene carbonate with a relatively small molecular diameter is useful. When carbon having been heat-treated at a temperature of 1500.degree. C. or less and slightly graphitizable carbon and the like, which are amorphous carbon having a large d002 of 3.40 angstroms or more, are used, propylene carbonate, .gamma.-butyrolactone, butylene carbonate and the like are useful.